When we speak the language of the human, we engage in a politics of inclusion; yet in offering our definitions of this human, we endorse something that serves to exclude. In any definition, the characteristics named as essential may be highly idealised, sometimes verging on the imaginary, but even when they capture something real the selections made are a matter of history and politics. As Ian Hacking has put it, we ‘make up people’. We do not make them up in the sense of conjuring them physically into existence, but when we decide that the crucial distinction is that between man and woman, or human and animal, or heterosexual and gay, we settle on definitions and boundaries that then mark our ways of thinking and living. The idea, for example, that humans divide into two sexes, male and female, is so much taken for granted that we tend to think of it as just a fact of nature – and it does indeed capture something ‘real’: a difference in our reproductive organs. But differentiating between humans on this basis was never the only possibility. We could, in principle, have decided that the key ‘natural’ distinction was between the short and the tall. Through much of history, people have claimed variations in skin colour and physiognomy as the key distinctions. So far as male and female is concerned, there may be no great mystery about why reproductive organs came to be viewed as such an important mode of differentiation – societies have to reproduce themselves, after all – but it is worth bearing in mind that this is not just ‘natural fact’. We should also bear this in mind when considering the human/non-human distinction.

As Felipe Fernandez-Armesto observes in So You Think You're Human?, St Francis of Assisi preached to ravens, St Antony of Padua reportedly administered communion to his horse, and in the – to most of us extraordinary – accounts of the animal trials of fifteenth-, sixteenth-, and seventeenth century Europe, we glimpse a very different division of the world in which animals seemed to have legal rights ‘practically on a par with humans’.